1. Field of the Invention
The present invention relates to a semiconductor laser device and, more particularly, to a semiconductor laser device which does not have a resonator comprising parallel planes.
2. Brief Description of the Prior Art
As is well known in the art, conventional semiconductor laser devices consist of a laser medium which provides gain and a resonator structure which provides a feedback necessary for a build-up of laser oscillation. The resonator structure consists of at least two plane surfaces which are parallel to each other. These plane surfaces are usually formed by cleaving the laser medium, such as GaAs, GaP, etc.
This kind of semiconductor laser, however, has drawbacks such that integration with other semiconductor elements, such as transistors, diodes, etc. in one semiconductor body becomes very difficult, since the cleavage technique should be introduced to form the cavity resonator, and that a control of a wavelength generated therefrom becomes difficult, since the wavelength is determined by the length of the resonator which is defined by cleaved surfaces of the laser medium and it is very difficult to provide an accurate distance between cleaved surfaces.
In the field of semiconductor laser devices, it, therefore, has been desired to provide a semiconductor laser device which can be integrated with other semiconductor elements and the wavelength of which can be easily controlled.
There has been proposed a new type of a semiconductor laser device which is called a distributed feedback laser and which is capable to satisfy the above-mentioned needs ("Applied Physics Letters", Vol. 18, No. 4, Feb. 1971, pp. 152-154), and this semiconductor laser device has been developed and reported in "Applied Physics Letters", Vol. 22, No. 10, May 1973, pp. 515-516, and "Applied Physics Letters", Vol. 23, No. 5, September, 1973, pp. 224-225.
The distributed feedback laser has a structure such that a surface of a laser active layer is periodically corrugated.
At the corrugated surface, however, there are many non-radiative recombination centers created during the fabrication of the corrugation at the surface of the laser active layer. Therefore, the distributed feedback laser has such a drawback that a threshold value for laser oscillation becomes high.